C9H8 Pyrolysis. o-Tolylacetylene, Indene, 1-Indenyl, and Biindenyls and the Mechanism of Indene Pyrolysis.

Abstract

o-Tolylacetylene 5 is obtained by flash vacuum pyrolysis (FVP) of the isoxazolone 13a at 800 °C/10(-4) hPa. At 900-1000 °C the acetylene 5 isomerizes to indene 1, which reacts further by elimination of a hydrogen atom and dimerization of the 1-indenyl radical 9 to 1,1'-biindenyl 10. The latter undergoes partial isomerization to 3,3'-biindenyl 16, and further pyrolysis of the biindenyls yields higher polycyclic aromatic hydrocarbons (PAHs), particularly chrysene 2. C-H bond breakage in indene, which occurs with an activation energy of 80 ± 5 kcal/mol with formation of the 1-indenyl radical 9, has been the subject of much investigation in relation to hydrocarbon combustion, in particular the formation of chrysene and other PAHs from indene, which itself is formed in the combustion of toluene and other hydrocarbons. However, C-C bond breakage also needs to be considered. Calculations at the B3LYP/6-311+G(d,p) level indicate that key C-C bond breakages in indene have free energies of activation of ca. 80 kcal/mol. Positive entropies of activation make all these reactions more facile at high temperatures relevant to hydrocarbon combustion chemistry. C1-C2 bond breakage results in the formation of o-tolylvinylidene 6 and o-tolylacetylene 5. The reversible 1,2-shift interconverting 5 and 6 (the Roger Brown rearrangement) can lead to carbon scrambling in C3-labeled indene 1a, resulting in indene 1d carrying the label in positions 1, 2, and 3 and explaining the (14)C-labeling pattern observed by Badger et al. in the derived chrysene 2d. o-Tolylacetylene 5 and o-tolylvinylidene 6 should be considered as intermediates in models of the fuel-rich combustion of toluene, indene, and other hydrocarbons.